Phosphoric acid resistant polymaleimide prepolymer compositions

ABSTRACT

The present invention relates to a phosphoric acid resistant advanced polymaleimide prepolymer composition including a polymaleimide prepolymer resulting from the advancement reaction of a polyimide and an alkenylphenol, alkenylphenol ether or mixture thereof in the presence of an amine catalyst; a solvent and a heteropolyacid. The phosphoric acid resistant advanced polymaleimide prepolymer composition may be used in various applications such as prepregs, laminates, printed circuit boards, castings, composites, molded articles, adhesives and coatings.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 13/256,269, pending, which is the National Phase ofInternational Application PCT/US2010/027418 filed Mar. 16, 2010, whichdesignated the U.S. and which was published in English and which claimspriority to U.S. Pat. App. Ser. No. 61/160,830 filed Mar. 17, 2009. Thenoted applications are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

FIELD OF THE INVENTION

The present invention is directed to phosphoric acid resistant advancedpolymaleimde prepolymer compositions and products, such as prepregproducts, produced therefrom. The phosphoric acid resistant advancedpolymaleimide prepolymer compositions include: (i) a polymaleimideprepolymer resulting from the advancement reaction of a polyimide and analkenylphenol or alkenylphenol ether in the presence of an aminecatalyst; (ii) a solvent; and (iii) a heteropolyacid.

BACKGROUND OF THE INVENTION

Bimaleimides may be used as raw materials for the manufacture ofpolyaddition and polymerization products. For example, U.S. Pat. No.3,658,764 discloses polyaddition products produced from unsaturatedbismaleimides and amines; U.S. Pat. No. 3,741,942 describes themanufacture of polyaddition products from bismaleimides and organicdithiols; U.S. Pat. Nos. 4,038,251 and 4,065,433 describe the reactionof bismaleimides with polyhydric phenols and polyhydric alcohols in thepresence of a catalyst to form the polyaddition products; and U.S. Pat.No. 4,100,140 describes polyaddition products prepared frombismaleimides and alkenylphenols or alkenylphenol ethers.

Because of their thermal stability and good mechanical properties,polyaddition products produced from bismaleimides may be used in variousprepreg applications. One such polyaddition product, supplied as apowder, is the reaction product of bismaleimidodiphenyl methane andmethylene dianiline. The powder polyaddition product may then bedissolved in a high boiling solvent, such as N-methylpyrrolidone, forprepreg use. The stability of such a solution however is limited due torapidly occurring precipitation and viscosity increase upon standing.Therefore, formulation of the solution must occur immediately prior touse.

Improvements have been noted in the polyaddition products and solutionsdisclosed in U.S. Pat. Nos. 5,189,128 and 5,637,387. U.S. Pat. No.5,189,128 discloses a polyaddition product produced from the reaction ofpolymaleimide and alkenylphenol or alkenylphenol ether in the presenceof specific molar amounts of basic catalyst for certain reaction timesand at certain reaction temperatures. The resulting product exhibitsimproved solubility in the lower boiling solvent methyl ethyl ketone andimproved storage stability as evidenced by an absence of precipitation.A further improvement in storage stability is described in U.S. Pat. No.5,637,387 where phenothiazine or hydroquinone is added after thepolymaleimide has reacted with the alkenylphenol or alkenylphenol ether.

It is an object of the present invention to provide an improvedpolyaddition product composition which exhibits resistance to acids, andin particular, phosphoric acid, without any adverse impact on thethermal and mechanical properties of the prepreg products madetherefrom.

SUMMARY OF THE INVENTION

The present invention relates to a phosphoric acid resistant advancedpolymaleimide prepolymer composition comprising:

-   -   (a) a polymaleimide prepolymer resulting from the advancement        reaction of a polyimide and an alkenylphenol, alkenylphenol        ether or mixture thereof in the presence of an amine catalyst;    -   (b) a solvent; and    -   (c) a heteropolyacid.

The phosphoric acid resistant advanced polymaleimide prepolymercomposition may be prepared by reacting at an elevated temperature apolyimide and an alkenylphenol, an alkenylphenol ether or mixturethereof, in the presence of an amine catalyst to form the polymaleimideprepolymer, substantially removing all of the amine catalyst; andsubsequently adding a solvent and heteropolyacid. In another embodiment,a prepreg or laminate structure may be prepared by curing a fabric orfiber impregnated with the phosphoric acid resistant advancedpolymaleimide prepolymer composition of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The phosphoric acid resistant advanced polymaleimide prepolymercomposition generally includes (i) a polymaleimide prepolymer resultingfrom the advancement reaction of a polyimide and an alkenylphenol,alkenylphenol ether or mixture thereof in the presence of an aminecatalyst; (ii) a solvent; and (iii) a heteropolyacid. It has beensurprisingly discovered that the addition of solvent and heteropolyacidto the polymaleimde prepolymer allows the prepolymer to exhibitresistance to acidic environments, especially highly concentratedphosphoric acidic environments. The phosphoric acid resistant advancedpolymaleimide prepolymer composition of the present invention may beexposed to acidic environments at room temperature or temperatures evenhigher than room temperature for a prolonged period of time withoutadverse effects. By “room temperature” it is meant a temperature ofabout 20° C.

Applicable polyimides contain at least two radicals of the formula

where R¹ is hydrogen or methyl.

In another embodiment, the polyimide is a polymaleimide, preferably abismaleimide of the formula

where R¹ is hydrogen or methyl and X is —C_(n)H_(2n)— with n=2 to 20,—CH₂CH₂SCH₂CH₂—, phenylene, naphthalene, xylene, cyclopentylene,1,5,5-trimethyl-1,3-cyclohexylene, 1,4-cyclohexylene,1,4-bis-(methylene)-cyclohexylene, or groups of the formula (a)

where R² and R³ independently are chlorine, bromine, methyl, ethyl, orhydrogen and Z is a direct bond, methylene, 2,2-propylidene, —CO—, —O—,—S—, —SO— or —SO₂—. Preferably, R¹ is methyl, X is hexamethylene,trimethylhexamethylene, 1,5,5-trimethyl-1,3-cyclohexylene or a group ofthe indicated formula (a) in which Z is methylene, 2,2-propylidene or—O— and R² and R³ are hydrogen.

Examples of polyimides include: N,N′-ethylene-bismaleimide,N,N′-hexamethylene-bismaleimide, N,N′-m-phenylene-bismaleimide,N,N′-p-phenylene-bismaleimide, N,N′-4,4′-diphenylmethane-bismaleimide,N,N′-4,4′-3,3′-dichloro-diphenylmethane-bismaleimide,N,N′-4,4′-(diphenylether)-bismaleimide,N,N′-4,4′-di-phenylsulphone-bismaleimide,N,N′-4,4′-dicyclohexylmethane-bismaleimide,N,N′-α,4,4′-dimethylenecyclohexane-bismaleimide,N,N′-m-xylene-bismaleimide, N,N′-p-xylene-bismaleimide,N,N′-4,4′-diphenylcyclohexane-bismaleimide,N,N′-m-phenylene-biscitraconimide,N,N′-4,4′-diphenylmethane-biscitraconimide,N,N′-4,4′-2,2-diphenylpropane-bismaleimide, N,N′-α,1,3-dipropylene-5,5-dimethyl-hydantoin-bismaleimide,N,N′-4,4′-diphenylmethane-bisitaconimide,N,N′-p-phenylene-bisitaconimide,N,N′-4,4′-diphenylmethane-bisdimethylmaleimide,N,N′-4,4′-2,2′-diphenylpropane-bisdimethylmaleimide,N,N′-hexamethylene-bisdimethylmaleimide,N,N′-4,4′-(diphenylether)-bisdimethylmaleimide andN,N′-4,4′-diphenylsulphone-bisdimethylmaleimide.

Applicable alkenylphenols and alkenylphenol ethers may includeallylphenols, methallylphenols or ethers thereof. Preferably, thealkenylphenol and alkenylphenol ether is a compound of the formulae(1)-(4):

where R is a direct bond, methylene, ispopropylidene, —O—, —S—, —SO— or—SO₂—;

where R⁴, R⁵ and R⁶ are each independently hydrogen or a C₂-C₁₀ alkenyl,preferably an allyl or propenyl, with the proviso that at least one ofR⁴, R⁵ or R⁶ is a C₂-C₁₀ alkenyl;

where R⁴, R⁵, R⁶ and R⁷ are each independently hydrogen or a C₂-C₁₀alkenyl, preferably an allyl or alkenyl, with the proviso that at leastone of R⁴, R⁵, R⁶ or R⁷ is a C₂-C₁₀ alkenyl and R is defined as informula (1) and

where R⁸, R⁹, R¹⁰, R¹¹, R¹² and R¹³ are each independently hydrogen,C₁-C₄ alkyl, and C₂-C₁₀ alkenyl, preferably allyl or propenyl, with theproviso that at least one of R⁸, R⁹, R¹⁰, R¹¹, R¹² and R¹³ is a C₂-C₁₀alkenyl and b is an integer from 0 to 10. It is also possible to usemixtures of compounds of the formulae (1)-(4).

Examples of alkenylphenol and alkenylphenol ether compounds include:O,O′-diallyl-bisphenol A, 4,4′-dihydroxy-3,3′-diallyldiphenyl,bis(4-hydroxy-3-allylphenyl)methane,2,2-bis(4-hydroxy-3,5-diallylphenyl)propane, O,O′-dimethallyl-bisphenolA, 4,4′-dihydroxy-3,3′-dimethallyldiphenyl,bis(4-hydroxy-3-methallylphenyl)methane,2,2-bis(4-hydroxy-3,5-dimethallylphenyl)-propane,4-methallyl-2-methoxyphenol, 2,2-bis(4-methoxy-3-allylphenyl)propane,2,2-bis(4-methoxy-3-methallylphenyl)propane,4,4′-dimethoxy-3,3′-diallyldiphenyl,4,4′-dimethoxy-3,3′-dimethallyldiphenyl,bis(4-methoxy-3-allylphenyl)methane,bis(4-methoxy-3-methallylphenyl)methane,2,2-bis(4-methoxy-3,5-diallylphenyl)propane,2,2-bis(4-methoxy-3,5-dimethallylphenyl)propane, 4-allylveratrole and4-methallyl-veratrole.

The alkenylphenol, alkenylphenol ether or mixture thereof may beemployed in a range of between about 0.05 moles-2.0 moles per mole ofpolyimide. In another embodiment, the alkenylphenol, alkenylphenol etheror mixture thereof may be employed in a range of between about 0.1moles-1.0 mole per mole of polyimide.

Applicable amine catalysts include tertiary, secondary and primaryamines or amines which contain several amino groups of different typesand quaternary ammonium compounds. The amines may be either monoaminesor polyamines and may include: diethylamine, tripropylamine,tributylamine, triethylamine, triamylamine, benzylamine,tetramethyl-diaminodiphenylmethane, N,N-diisobutylaminoacetonitrile,N,N-dibutylaminoacetonitrile, heterocyclic bases, such as quinoline,N-methylpyrrolidine, imidazole, benzimidazole and their homologues, andalso mercaptobenzothiazole. Examples of suitable quaternary ammoniumcompounds which may be mentioned are benzyltrimethylammonium hydroxideand benzyltrimethylammonium methoxide. Tripropylamine is preferred.

The amine catalyst may be employed in a range of between about 0.1%-10%by weight of amine catalyst per total weight of the advancementreactants. In another embodiment, the amine catalyst present may beemployed in a range of between about 0.2%-5% by weight of amine catalystper total weight of the advancement reactants.

The method of preparation of the polymaleimide prepolymer includesblending the polyimide and the alkenylphenol, alkenylphenol ether ormixture thereof and heating the blend to a temperature of about 25°C.-150° C. until a clear melt is obtained. The amine catalyst may thenbe added and the reaction continued for an appropriate amount of time ata temperature of about 100° C.-140° C. whereupon all of the aminecatalyst is removed under vacuum. The degree of advancement may bemonitored by measuring resin melt viscosity using a 0-100 poise scale at125° C. and may range from 20-90 poise for the advanced polymaleimideprepolymer. Gel time may also be used as an additional parameter andreflects the time to total gel formation as determined at a temperatureof about 170° C.-175° C. and may range from 300 seconds-2000 seconds.

The phosphoric acid resistant advanced polymaleimide prepolymercomposition further includes a solvent. The solvent may be a low boilingsolvent (boiling points up to about 160° C. and preferably up to about100° C.) including ketones such as acetone, methyl ethyl ketone andmethyl isobutyl ketone; glycol ethers and glycol ether acetates such aspropylene glycol monomethyl ether, propylene glycol methyl etheracetate, ethylene glycol methyl ether, ethylene glycol ethyl ether andglycol ethyl ether acetate; hydrocarbons such as toluene and anisole;methoxy propanol; dimethylformamide; and mixtures thereof.

The solvent may be employed in a range of between about 10%-50% byweight based on the total weight of the phosphoric acid resistantadvanced polymaleimide prepolymer composition. In another embodiment,the solvent may be employed in a range of between about 17.5%-40% byweight, preferably between about 20%-30% by weight, based on the totalweight of the advanced polymaleimide prepolymer composition.

The phosphoric acid resistant advanced polymaleimide prepolymercomposition further includes a heteropolyacid. The heteropolyacid is anoxygen acid with P, As, Si, or B as central atoms which are connectedwith W, Mo or V via oxygen bridges. Examples of such acids are tungstenand molybdenum phosphoric acid and tungsten and molybdenum arsenic acid.Preferred is phosphotungstic acid hydrate.

The heteropolyacid may be employed in a range of between about0.0001%-10% by weight based on the total weight of the phosphoric acidresistant advanced polymaleimide prepolymer composition. In anotherembodiment, the heteropolyacid may be combined with the solvent as a1%-5% solution which is then employed in a range of between about 1%-5%by weight, based on the total weight of the phosphoric acid resistantadvanced polymaleimide prepolymer composition.

The phosphoric acid resistant advanced polymaleimide prepolymercomposition may be prepared by adding the solvent and heteropolyacid tothe polymaleimide prepolymer at the conclusion of the advancementreaction. High solids, up to about 80% by weight, and low viscositycompositions of 50 centipoise or less are thereby formed which are acidresistant for days at room temperature or temperatures higher than roomtemperature.

In addition to the components above, the phosphoric acid resistantadvanced polymaleimide prepolymer composition may optionally be mixed atany stage before cure with one or more stabilizers, extenders, fillers,reinforcing agents, pigments, dyestuffs, plasticizers, tackifiers,rubbers, accelerators, diluents or any mixture thereof.

Stabilizers which may be employed include: phenothiazine itself orC-substituted phenothiazines having 1 to 3 substituents or N-substitutedphenothiazines having one substituent for example,3-methyl-phenothiazine, 3-ethyl-phenothiazine, 10-methyl-phenothiazine;3-phenyl-phenothiazine, 3,7-diphenyl-phenothiazine;3-chlorophenothiazine, 2-chlorophenothiazine, 3-bromophenothiazine;3-nitrophenothiazine, 3-aminophenothiazine, 3,7-diaminophenothiazine;3-sulfonyl-phenothiazine, 3,7-disulfonyl-phenothiazine,3,7-dithiocyanatophenthiazin; substituted quinines and catechols, coppernaphthenate, zinc-dimethyldithiocarbonate and phosphotungistic acidhydrate. The stabilizers may be added to the phosphoric acid resistantadvanced polymaleimide prepolymer composition in amounts of about0.1%-10% by weight based on the total weight of advanced polymaleimideprepolymer composition.

Extenders, reinforcing agents, fillers accelerators and pigments whichcan be employed include, for example: coal tar, bitumen, glass fibers,boron fibers, carbon fibers, cellulose, polyethylene powder,polypropylene powder, mica, asbestos, quartz powder, gypsum, antimonytrioxide, bentones, silica aerogel (“aerosil”), lithopone, barite,titanium dioxide, eugenol, dicummyl peroxide, isoeugenol, carbon black,graphite, and iron powder. It is also possible to add other additives,for example, flameproofing agents, flow control agents such assilicones, cellulose acetate butyrate, polyvinyl butyrate, waxes,stearates and the like (which are in part also used as mold releaseagents) to the advanced polymaleimide prepolymer composition.

The phosphoric acid resistant advanced polymaleimide prepolymercompositions described above are suitable in a broad range of end usessuch as in prepregs, laminates of various thicknesses, printed circuitboards, castings, composites, moulded articles, adhesives and coatings.

A prepreg may by obtained by impregnating or coating a base materialwith the phosphoric acid resistant advanced polymaleimide prepolymercomposition. The base material includes all base materials used forlaminates. Examples thereof include various glass cloth such as E glasscloth, NE glass cloth and D glass cloth, natural inorganic fiberfabrics, woven fabrics and nonwoven fabrics obtained from liquid crystalfibers such as an aromatic polyamide fiber or an aromatic polyesterfiber, woven fabrics and nonwoven fabrics obtained from synthetic fiberssuch as a polyvinyl alcohol fiber, a polyester fiber or an acrylicfiber, natural fiber nonwoven fabrics such as cotton fabric, linenfabric or felt, a carbon fiber fabric, natural cellulose type fabricssuch as kraft paper, cotton paper or paper-glass combined paper, andporous PTFE.

In one embodiment, a polymer base material is impregnated or coated withthe advanced polymaleimide prepolymer composition of the presentinvention. The polymer base material is not specially limited so long asit is a woven fabric, a nonwoven fabric, a sheet or a porous body eachof which uses a polymer. Examples thereof include liquid crystalpolymers such as lyotropic liquid crystal polymers typified by aromaticpolyamide, polyphenylene benzothiazole, thermotropic liquid crystalpolymers typified by aromatic polyester, polyesteramide, a polyamide, anaramid resin, polyphenylene ether, polyphenylene sulfide, polyethylene,polypropylene, and a fluororesin. The polymer is properly selecteddepending upon an intended application or performance as required. Thesepolymers may be used alone or in combination as required. The thicknessof the base material is not specially limited. Generally, it is about 3μm to 200 μm.

The process for producing the prepreg is not specially limited so longas it can combine the phosphoric acid resistant advanced polymaleimideprepolymer composition and a base material to produce the prepreg. Inone embodiment, a method is provided in which the above describedadvanced polymaleimide prepolymer composition is impregnated or appliedto the base material and then heated, for example, in a dryer, at 80° C.to 200° C. for 1 to 90 minutes to B-stage the composition and therebyproduce prepreg. The resin content of the prepreg may range from about30%-90% by weight.

A metal-clad laminate may also be obtained by stacking one prepreg or atleast two prepregs, laminating metal foil(s), such as copper foil oraluminum foil, on upper and lower surfaces or one surface of the stackedprepreg(s) and heating and pressurizing the resultant set.

General techniques of a laminate and a multilayer board for printedwiring boards may be applied for the molding conditions of themetal-clad laminate. For example, generally, a multiplaten press, amultiplaten vacuum press, continuous molding, an autoclave moldingmachine or the like is used at a temperature of between about 100°C.-300° C. and a pressure of between about 0.2 MPa-10 MPa, and at aheating time of about 0.1-5 hours. Further, it is also possible toproduce a multilayer board by combining the prepreg of the presentinvention with an internal layer wiring board which is separatelyprepared and laminate-molding the resultant set.

In a particular embodiment, a prepreg or laminate structure comprisesthe cured product of a base material impregnated or coated with aphosphoric acid resistant advanced polymaleimide prepolymer compositioncomprising (i) a polymaleimide prepolymer resulting from the advancementreaction of a polyimide and an alkenylphenol, alkenylphenol ether ormixture thereof in the presence of an amine catalyst; (ii) a solvent;and (iii) a heteropolyacid.

EXAMPLES Example 1

This example illustrates the preparation of a phosphoric acid resistantadvanced polymaleimide prepolymer composition of the present invention.

Formulation Parts By Weight N,N′-4,4′-diphenylmethane bismaleimide 40-454,4′-(methylethylidene)bis(2-propenyl)phenol 30-35 Tripropylamine0.1-1.0 Solvent 20-30 Phosphotungstic acid hydrate 0.1-1.0

4,4′-(methylethylidene)bis(2-propenyl)phenol was charged to a reactorand placed under vacuum. N,N′-4,4′-diphenylmethane bismaleimide wasadded to the reactor and mixed with the4,4′-(methylethylidene)bis(2-propenyl)phenol to form a reaction mixture.The reaction mixture was heated to 132° C. and then placed under vacuum,and upon becoming a clear amber solution, was then cooled to 100° C.Tripropylamine was added to the solution and continuously stirred for 30minutes. Thereafter, the temperature was increased to and maintained at120° C. and the solution placed under vacuum for 1.5 hours. The vacuumwas removed and the solvent was added to the solution and thetemperature was maintained at a temperature of 120° C. until a viscosityof 70-80 poise was reached. The solution was then allowed to cool to 50°C. whereupon phosphotungistic acid hydrate was added. The advancedpolymaleimide prepolymer composition which was then discharged from thereactor and cured at 177° C. for 90 minutes then 218° C. for four hours.

Samples of the cured advanced polymaleimide prepolymer composition werethen stored at 200° C. and exposed to phosphoric acid for a period oftime and the dimensional change and weight loss were measured. Theresults are presented in Table 1:

TABLE 1 % Change Days In Storage % Change X % Change Y % Change Z Weight1 0.00 −0.57 0.00 −0.27 7 0.00 0.00 0.00 0.05 31 0.00 0.00 0.00 2.40

Example 2

A formulation stability study for Example 1 was performed at roomtemperature and at −4° C. The blended formulation's gel time wasinitially measured at 171° C. and its viscosity was measured at roomtemperature. The formulation was then monitored every month until gel.During the study, samples of the formulation were pulled and allowed toadjust to room temperature before the stability data was collected. Whenstored at −4° C., the formulation demonstrated stability over a 12 monthperiod and thus can be stored at −4° C. for at least one year. Incomparison, when stored at room temperature, the formulationdemonstrated stability over a 6 month period.

The above-disclosed subject matter is to be considered illustrative, andnot restrictive, and the appended claims are intended to cover all suchmodifications, enhancements, and other embodiments, which fall withinthe true scope of the present invention. Thus, to the maximum extentallowed by law, the scope of the present invention is to be determinedby the broadest permissible interpretation of the following claims andtheir equivalents, and shall not be restricted or limited by theforegoing detailed description.

What is claimed is:
 1. A phosphoric acid resistant advancedpolymaleimide prepolymer composition comprising: (a) a polymaleimideprepolymer resulting from the advancement reaction of a polyimidecontaining at least two radicals of the formula

where R¹ is hydrogen or methyl and an alkenylphenol, alkenylphenol etheror mixture thereof in the presence of an amine catalyst; (b) 20-30 partsby weight, based on the total weight of the composition, of a solvent;and (c) 0.1-1.0 parts by weight, based on the total weight of thecomposition, of a heteropolyacid wherein the solvent and theheteropolyacid are added to the polymaleimide prepolymer at theconclusion of the advancement reaction and the phosphoric acid resistantadvanced polymaleimide prepolymer composition has a viscosity of 50centipoise or less.
 2. The phosphoric acid resistant advancedpolymaleimide prepolymer composition of claim 1 wherein the polyimide isa bismaleimide of the formula

where R¹ is hydrogen or methyl and X is C_(n)H_(2n)— with n=2,—CH₂CH₂SCH₂CH₂—, phenylene, naphthalene, xylene, cyclopentylene,1,5,5-trimethyl-1,3-cyclohexylene, 1,4-cyclohexylene,1,4-bis-(methylene)-cyclohexylene, or groups of the formula (a)

where R² and R³ independently are chlorine, bromine, methyl, ethyl, orhydrogen and Z is a direct bond, methylene, 2,2-propylidene, —CO—, —O—,—S—, —SO— or —SO₂—.
 3. The phosphoric acid resistant advancedpolymaleimide prepolymer composition of claim 2 wherein R¹ is methyl, Xis hexamethylene, trimethylhexamethylene,1,5,5-trimethyl-1,3-cyclohexylene or a group of the formula (a) in whichZ is methylene, 2,2-propylidene or —O— and R² and R³ are hydrogen. 4.The phosphoric acid resistant advanced polymaleimide prepolymercomposition of claim 2 wherein the bismaleimide isN,N′-4,4′-diphenylmethane bismaleimide.
 5. The phosphoric acid resistantadvanced polymaleimide prepolymer composition of claim 1 wherein thealkenylphenol or alkenylphenol ether is a compound of the formulae(1)-(4):

where R is a direct bond, methylene, ispopropylidene, —O—, —S—, —SO— or—SO₂—;

where R⁴, R⁵ and R⁶ are each independently hydrogen or a C₂-C₁₀ alkenylwith the proviso that at least one of R⁴, R⁵ or R⁶ is a C₂-C₁₀ alkenyl;

where R⁴, R⁵, R⁶ and R⁷ are each independently hydrogen or a C₂-C₁₀alkenyl with the proviso that at least one of R⁴, R⁵, R⁶ or R⁷ are is aC₂-C₁₀ alkenyl and R is defined as in formula (1); and

where R⁸, R⁹, R¹⁰, R¹¹, R¹² and R¹³ are each independently hydrogen,C₁-C₄ alkyl, and C₂-C₁₀ alkenyl with the proviso that at least one ofR⁸, R⁹, R¹⁰, R¹¹, R¹² and R¹³ is a C₂-C₁₀ alkenyl and b is an integerfrom 0 to
 10. 6. The phosphoric acid resistant advanced polymaleimideprepolymer composition of claim 1 wherein the heteropolyacid isphosphotungstic acid hydrate.
 7. The phosphoric acid resistant advancedpolymaleimide prepolymer composition of claim 1 wherein the solvent isacetone.
 8. A prepreg or laminate structure comprising the cured productof a fabric or fiber impregnated or coated with a phosphoric acidresistant advanced polymaleimide prepolymer composition comprising: (a)a polymaleimide prepolymer resulting from the advancement reaction of apolyimide containing at least two radicals of the formula

where R¹ is hydrogen or methyl and an alkenylphenol, alkenylphenol etheror mixture thereof in the presence of an amine catalyst; (b) 20-30 partsby weight, based on the total weight of the composition, of a solvent;and (c) 0.1-1.0 parts by weight, based on the total weight of thecomposition, of a heteropolyacid wherein the solvent and theheteropolyacid are added to the polymaleimide prepolymer at theconclusion of the advancement reaction and the phosphoric acid resistantadvanced polymaleimide prepolymer composition has a viscosity of 50centipoise or less.